The metabolism of methaqualone (2-methyl-3-o-tolyl-4(3H)-quinazolinone) has been studied in man and the rat using gas phase analytical methods. Seven new metabolites formed by the epoxide-diol pathway were detected in human urine after methaqualone administration. Five of these compounds were characterized as dihydrodiols and two as hydroxydihydrodiols. The seven dihydrodiol metabolites were present in the nonhydrolyzed fraction isolated from urine. After intraperitoneal administration of methaqualone to the rat (40 mg/kg) the major monohydroxyl metabolites of the drug in hydrolyzed urine were identified as 2-methyl-3-(2'-hydroxymethylphenyl)-4(3H)-quinazolinone (I) and 2-hydroxymethyl-3-o-tolyl-4(3H)-quinazolinone (II). Two dihydroxyl metabolites were also present, but only trace amounts of a dihydrodiol were detected. The major monohydroxyl metabolites of methaqualone detected in human urine after enzymic hydrolysis were I, II, 2-methyl-3-(3'-hydroxy-2'-methylphenyl)-4(3H)-quinazolinone (III), and 2-methyl-3-(4'-hydroxy-2'-methylphenyl)-4(3H)-quinazolinone (IV). Hydroxylation of the tolyl moiety of methaqualone probably occurs by way of an epoxide intermediate. The phenols, III and IV, may be formed from an epoxide or from the dihydrodiol(s) by enzymic or nonenzymic reactions. The results obtained suggest that epoxidation of methaqualone represents a major pathway of metabolism in the human.